This is a tremendous site with free videos and exercises covering basic arithmetic through calculus. A good place to go if your skills need some refreshing.

Finite Mathematics

Matrix Reduction Practice - Try out this page for practice in matrix row operations. The page can prompt you to do one operation at a time, or you can reduce an entire matrix at once. (Still in development, but reasonably stable.)

College Algebra, Precalculus, and Calculus

CalcChat - Worked-out answers to odd-numbered problems in the Larson, Hofstetler texts. Worth checking when you get stuck.

Calculus, by Gilbert Strang - A complete textbook made available through MIT's Open Courseware program.

MIT Open Courseware - Massachusetts Institute of Technology posts lots and lots of lecture notes and exercises. Neat resource! Their mathematics courses include many undergraduate and graduate courses.